1. Field of the Invention
The present invention relates to an inkjet printer designed to print color images comprised of process colors as defined by the subtractive color model, and more particularly to an inkjet printer capable of reducing and/or eliminating chromatic variation in adjacent print swaths when printing in a bidirectional mode.
2. Description of the Related Art
There have been known such output devices of inkjet, laser beam, thermal, and thermal transfer types, as printers for computers and word processors and raster plotters for CAD systems in the art.
Among those, an inkjet printer is possible to print a high-precision image at a high-speed by firing inks on a print medium such as paper from a print head. The inkjet printers have grown popular for the public use along with the current widespread use of computers. The most employed color printers are such types that are capable of firing several color inks from one print head. In particular, they can be used mostly for printing images with multi-color/multi-tone processed by the computers.
In such the inkjet printer, the print head is scanned in a direction across a print paper (the main scan direction) in order to print a printable region per scan. At the same time, the print paper is advanced in a direction perpendicular to the main scan direction (the sub scan direction). The print head generally comprises a plurality of head-segments arrayed in the main scan direction. Each head-segment responds to each ink color. Each head-segment has a plurality of nozzles arranged at different locations in the sub scan direction. A color printing is performed in accordance with the subtractive color model. The subtractive color model is represented typically with a combination, CMY, of cyan (C), magenta (M) and yellow (Y) inks or a more common combination, CMYK, of CMY plus black (K) ink. There are various extensions such as CMYK plus light-density magenta (LM) and light-density cyan (LC), light-density black, and/or spot colors of orange, green, red and blue.
A common configuration would currently be a print head with four head-segments, one per color, arranged in a nozzle order of KCMY so that when printing in a unidirectional mode the K ink is the first to be placed on the print paper, followed by C, M, and finally Y ink.
The limitation of this design is that, should the printer be designed to print in a bidirectional mode, to improve overall print speed, each alternate print swath (the reverse print swath) would be created by placing the Y ink on the paper first, followed by M, C, and finally K ink contrarily to the forward print swath.
The result of this method of printing is a noticeable chromatic variation in adjacent print swaths, since a swath printed with an ink order of K, C, M, and Y would appear "lighter" to the human observer than a swath printed with an ink order of Y, M, C, and K. This phenomenon is due to the fact that each of the four standard subtractive process colors has a unique brightness distinguishable to the human eye.
The KCMY method of printing is based on the notion that optimum color reproduction is achieved with the subtractive color process by printing the darkest color, black (K), first followed by a brighter color than black, cyan(C), and so on. As an example, in a six-color system comprised of KCMY plus LC and LM, the LC and LM follow Y in the optimum order of lay down.
However, because both print speed and image quality must be balanced to achieve optimum commercial viability, along with responding to the demands from the market including a rapid shipment and cost-down, most inkjet printers support a bidirectional print mode, which has the effect of reducing print time by a factor of 25 to 30 percent compared to the unidirectional print mode.
This increase in print speed, however, can normally only be achieved by sacrificing image quality, specifically a noticeable "banding" that occurs in parts of, or on occasion throughout the entire image. This phenomenon can be reduced by interleaving print swaths, but cannot be entirely eliminated.
FIGS. 9A-C illustrate a theoretical model of an interleaved print swath using a print head with a vertical dot pitch of 1/18.sup.th inch, printing with a horizontal resolution of 360 dots-per-inch (dpi).
As shown in FIG. 9A, when a print head 100 travels forward on a first pass (shown by an arrow R1) in the main scan direction first, ink nozzles 101 mounted on the print head 100 fire inks, creating a printed part with a horizontal resolution of 360 dpi and a vertical resolution of 180 dpi. In this forward print operation, all dots are printed in KCMY order: the brightest color is printed finally.
The print head 100 is then stepped a certain distance (for example, a 1/2-tall print swath) down in the sub scan direction as shown in FIG. 9B, and the print head 100 travels reverse on a second pass in the main scan direction. At the same time, inks are fired from the ink nozzles 101 to create a printed part with a horizontal resolution of 360 dpi and a vertical resolution of 180 dpi. As a result of these forward and reverse print operations, a 1/2-tall full dot print swath SWT1 is created with both horizontal and vertical resolutions of 360 dpi. In this reverse print operation, all dots are printed in YMCK order: the darkest color is printed finally.
The print head 100 is further stepped a certain distance down in the sub scan direction as shown in FIG. 9C, the print head 100 travels on the first pass again (shown by an arrow R2). At the same time, inks are fired from the ink nozzles 101 to create a printed part with a horizontal resolution of 360 dpi and a vertical resolution of 180 dpi. As a result of these reverse and forward print operations, another 1/2-tall full dot print swath SWT2 is created with both horizontal and vertical resolutions of 360 dpi. In this forward print operation, all dots are printed in KCMY order: the brightest color is printed again finally.
A study of the theoretical model illustrated above would indicate that interleaving each print swath would eliminate chromatic variation in adjacent print swaths, since each swath would consist of an equal number of vertically interlaced dots of alternating density. However, the above model does not take into account the phenomenon of dot gain, which results in a small overlapping of adjacent dots.
Dot gain occurs when an ink droplet of a given size increases in diameter as it dries on the substrate surface. This mechanism is necessary to ensure optimum image quality and color saturation; without adequate dot gain, a printed image will appear "washed out," since too much of the underlying surface (typically white in color) would show through between the gaps in the dots.
FIG. 10 details the dot gain in the above theoretical model.
As shown in FIG. 10A, when the print head 100 performs the reverse operation, low-brightness dots D2 are laid on top of high-brightness dots D1. Dot gain in this case gives "darker" impression to the human eye as seen from the printed result 110a. To the contrary, when the print head 100 performs the second forward operation as shown in FIG. 10B, high-brightness dots D1 are laid on top of low-brightness dots D2, resulting in "lighter" impression as seen from the printed result 110b. A complete printed image obtained through such the print operations can be observed darker in the swath SWT1 in case of right-to-left operations (L1, L2, . . . , Ln) performed by the print head, and lighter in the swath SWT2 in case of left-to-right operations (R1, R2, . . . , Rn). Higher vertical resolution is often achieved by tighter interleaving of each print swath, chromatic variations tend to become less noticeable on higher resolution printers. However, the degree of chromatic variation such as banding in adjacent print swaths remains the same.